Code review comment for lp:~osomon/webbrowser-app/thumbnails

Looks like there is a jenkins issue causing a failure, but I tested the functionality and it works well and looks good. Before we land this I'd like to understand the impact on memory this has to make sure it is acceptable. And I have a few other questions..

Specifically:
- roughly what is the memory usage associated with generating and loading each thumbnail
- how can we limit the thumbnail cache as this seems it will grow limitlessly as history grows. We probably need some sort of hard limit on disk size or number stored so we don't chew up the users disk space. And a way to recreate if one from the cache doesn't exist.
- Are all thumbnails for the history items loaded in memory when the browser is started?
- Or do we only load thumbnails for images that are visible on the screen? I hope so :)

We should ensure that if you have hundreds/thousands of history items (which is likely) that we're not creating and loading thumbnails for all of these up front but only on demand.